Seatpost setback and handling

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Luften
Posts: 125
Joined: Thu Sep 13, 2012 11:31 pm

by Luften

I've heard that the seatpost setback can affect the bike handling. Is this correct, and how does it change things?

I'm not talking about extremes, but just say going from a -25mm to a zero or vica-versa (and assuming the seat moves the same amount obviously).

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TobinHatesYou
Posts: 10102
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Luften wrote:
Tue Aug 16, 2022 10:38 am
I've heard that the seatpost setback can affect the bike handling. Is this correct, and how does it change things?

I'm not talking about extremes, but just say going from a -25mm to a zero or vica-versa (and assuming the seat moves the same amount obviously).

Let's say you move your saddle forward 25mm and make no other changes. Your saddle to grip reach will be 25mm shorter. You'll have to bend your arms more to get your back to the same angle as before. More weight will be over your front tire and less weight over your rear than before. More weight over the front-end might make it harder to ride perfectly straight lines. It also might make you lose traction on very steep climbs.

Now if you move your saddle backward, you might wheelie on steep climbs. You might track your bike better through loose gravel. It'll be easier to ride no-handed, you might be able to get lower on the bike without feeling scrunched up.

But you shouldn't do any of this. If you get a bike fit, your saddle x,y coordinates are precisely that. You move your saddle and your dynamic angles while pedaling all change.

biwa
Posts: 399
Joined: Fri Sep 21, 2018 8:39 pm

by biwa

You probably shouldn't make such a big change to your setback without going through a bike fit. It's going to noticeably affect your biomechanics and handling

BdaGhisallo
Posts: 2880
Joined: Wed Mar 17, 2004 1:38 pm

by BdaGhisallo

In and of itself, a setback post is not going to make a bit of difference if the other dimensions are controlled.

If you have a given wheelbase with given reach and stack numbers with constant chainstay and front center dimensions you won't notice a handling difference. If, within those fixed parameters, you go with a STA of 76 degrees and a 30mm setback seatpost or go with a zero setback post and a 72.5mm STA and they both put the saddle in the same place, you shouldn't feel any difference.

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wheelsONfire
Posts: 5023
Joined: Mon Jul 07, 2014 8:15 am
Location: NorthEU

by wheelsONfire

Weight distribution on the bike!

A longer stem will put you more forward so i will affect the steering input (it will be slower giving more stability)
Bikes:

Ax Lightness Vial EVO Race (2018.12.21)
viewtopic.php?f=10&t=156137
Paduano Racing Fidia (kind of shelved)
Open *UP* (2016.04.14)


Ex bike; Vial EVO D

Luften
Posts: 125
Joined: Thu Sep 13, 2012 11:31 pm

by Luften

Bike fits are variable. I have had 4-5 over the years and they are different depending on the fitter (with no noticeable change in my fitness or flexibility), so getting a bike fit is not a holy grail solution. At least for me. I've been happy with some, and equally unhappy with others.

And @BdaGhisallo obvisouly if the saddle is in the same place then it makes no difference! I did write in my post that the saddle would move with the setback change.

The 25mm to zero was just illustrative to discuss the handling change. I'm not contemplating this change personally. I'm contemplating a 1cm forward position, and either leaving the bars in the same position, or going 1cm longer on the stem. Maybe also the bars 1cm higher as well, and maybe some forward saddle tilt.
Last edited by Luften on Tue Aug 16, 2022 12:39 pm, edited 1 time in total.

pmprego
Posts: 2082
Joined: Mon Jan 21, 2019 3:16 pm

by pmprego

BdaGhisallo wrote:
Tue Aug 16, 2022 11:50 am
In and of itself, a setback post is not going to make a bit of difference if the other dimensions are controlled.

If you have a given wheelbase with given reach and stack numbers with constant chainstay and front center dimensions you won't notice a handling difference. If, within those fixed parameters, you go with a STA of 76 degrees and a 30mm setback seatpost or go with a zero setback post and a 72.5mm STA and they both put the saddle in the same place, you shouldn't feel any difference.
From the OP: "but just say going from a -25mm to a zero or vica-versa (and assuming the seat moves the same amount obviously)." - your answer does not abid for this condition. In your answer, you left the saddle in the same x,y coordinates thus nothing would change of course. That is not what the OP asked.

BdaGhisallo
Posts: 2880
Joined: Wed Mar 17, 2004 1:38 pm

by BdaGhisallo

pmprego wrote:
Tue Aug 16, 2022 12:07 pm
BdaGhisallo wrote:
Tue Aug 16, 2022 11:50 am
In and of itself, a setback post is not going to make a bit of difference if the other dimensions are controlled.

If you have a given wheelbase with given reach and stack numbers with constant chainstay and front center dimensions you won't notice a handling difference. If, within those fixed parameters, you go with a STA of 76 degrees and a 30mm setback seatpost or go with a zero setback post and a 72.5mm STA and they both put the saddle in the same place, you shouldn't feel any difference.
From the OP: "but just say going from a -25mm to a zero or vica-versa (and assuming the seat moves the same amount obviously)." - your answer does not abid for this condition. In your answer, you left the saddle in the same x,y coordinates thus nothing would change of course. That is not what the OP asked.
It was early. I guess I shouldn't trust my comprehension skills before the first coffee takes effect.

pmprego
Posts: 2082
Joined: Mon Jan 21, 2019 3:16 pm

by pmprego

BdaGhisallo wrote:
Tue Aug 16, 2022 1:44 pm
pmprego wrote:
Tue Aug 16, 2022 12:07 pm
BdaGhisallo wrote:
Tue Aug 16, 2022 11:50 am
In and of itself, a setback post is not going to make a bit of difference if the other dimensions are controlled.

If you have a given wheelbase with given reach and stack numbers with constant chainstay and front center dimensions you won't notice a handling difference. If, within those fixed parameters, you go with a STA of 76 degrees and a 30mm setback seatpost or go with a zero setback post and a 72.5mm STA and they both put the saddle in the same place, you shouldn't feel any difference.
From the OP: "but just say going from a -25mm to a zero or vica-versa (and assuming the seat moves the same amount obviously)." - your answer does not abid for this condition. In your answer, you left the saddle in the same x,y coordinates thus nothing would change of course. That is not what the OP asked.
It was early. I guess I shouldn't trust my comprehension skills before the first coffee takes effect.
:beerchug:

Hexsense
Posts: 2801
Joined: Wed Dec 30, 2015 12:41 am
Location: Louisiana, USA

by Hexsense

I hate when I accidentally found a tipping point of where I feel comfortable railing the corner hard. And when things feels off eventhough it shouldn't because it's such a small change.

On my old Cannondale CAAD10 48cm (and also Allez Sprint Disc). There is a tipping point about 1cm wide front or rear between me railling the corner without thinking. To the one that feel off enough that I have to pay attention and micro adjust my movements.
I don't like how it feel with short wheelbase, long stem, low seatpost offset. It felt as there's too much weight on front wheel and not enough grip on rear wheel.
I eventually just set the seat as forward as I wanted for pedaling but scoot myself back on back part of the saddle before the corner on these bikes.

The more modern Cannondale make my problem vanished. Both new SuperSix and SystemSix in small sizes have very long front center versus rear center length. In fact, my size 51 SystemSix has front center length of Specialized Tarmac size 56. So, I can set the seat as forward as I want, use long low stem. And yet, feel balanced in the middle of the bike without having to scoot back on rear part of the saddle.

Since chain stay length isn't size adjusted while front center is lengthen by size, I think anyone over size 52 won't experience what I feel.

by Weenie


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